Viruses, Viroids and Prions

Presented by

Tabitha Conner

Tasheba Conner

Viruses, Viroids and Prions

General Characteristics of Viruses

Viruses contain a single type of nucleic acid and a protein coat, sometimes enclosed by an envelope composed of lipids, proteins and carbohydrates.

Viruses are obligatory intracellular parasites. They multiply by using the host cell's synthesizing machinery to cause the synthesis of specialized elements that can transfer the viral nucleic acid to other cells.

• Host range refers to the spectrum of host cells in which a virus can multiply.
• Most viruses infect only specific types of cells in one host species.
• Host range is determined by the specific attachment site on the host cell's surface and the availability of host cellular factors.

• Viral size is ascertained by electron microscopy
• Viruses range from 20 to 1000 nm in length

Capsid and Enevelope

• The protein coat surrounding the nucleic acid of a virus is called the capsid
• The capsid is composed of subunits, capsomeres, which can be single type of protein or several types
• The capsid of some viruses is enclosed by an envelope consisting of lipids, proteins, and carbohydrates.
• Some envelopes are covered with carbohydrate protein complexes called spikes

Helical viruses resemble long rods and their capsids are hollow cylinders surrounding the nucleic acid

Nucleic Acid

Viroids and Prions

• Viruses contain either DNA or RNA, never both.
• The nucleic acid may be single or double stranded linear or circular, or divided into several separate molecules.

Polyhedral viruses are many sided

Complex viruses have complex structures

Viroids are infectious pieces of RNA that cause some plant diseases such as potato spindle tuber disease.

Viral Stucture

A virion is a complete fully developed viral particle composed of nucleic acid surrounded by a coat

Plant viruses must enter plant hosts through wounds or with invasive parasites such as insects.

Prions are infectious proteins

Prions diseases all involve the degeneration of brain tissues, such as mad cow disease

Viral Multiplication

Viruses do not contain enzymes for energy production or protein synthesis.

For multiplication, a virus must invade a host cell and direct the host's metabolic machinery to produce viral enzymes and components

Latent Viral Infections

• A latent viral infection is one in which the virus remains in the host cell for long periods without producing an infection
• Example would include cold sores and shingles.

Viruses and Cancer

• When activated, oncogenes transform normal cells into cancerous cells
• Viruses capable of producing tumors are called oncogenic viruses
• Several DNA viruses and retroviruses are oncogenic

Persistent Viral Infections

• Persistent viral infections are disease processes that occur over a long period and are generally fatal
• Persistent viral infections are caused by conventional viruses;viruses accumulate over a period of time.